Optical transmission, in which an information signal is modulated onto an optical carrier, is widely employed in modern communication systems. In particular, wide-area communications networks employ long-haul transmission links using single-mode optical fibres for the transmission of digital information at very high bit rates, using one or more optical carriers, or wavelengths, over each fibre. The distance over which data may be transmitted in single-mode optical fibres before some form of regeneration is required may be limited by optical attenuation and/or various dispersion mechanisms. The advent of practical optical amplifiers has substantially eliminated the loss limitation, particularly for systems operating in the third optical communications window at wavelengths around 1550 nm, in which erbium-doped fibre amplifiers are applicable.
Furthermore, linear dispersion processes, such as chromatic dispersion, may be compensated at any convenient point in a transmission system, and in principle to any desired degree of precision, using a variety of linear means. For example, applicable dispersion compensation techniques include the use of dispersion compensating fibre (DCF) and/or other dispersive optical elements selected and configured to provide an inverse dispersion characteristic to that of the transmission fibre. International patent application no. PCT/AU2006/001511 is directed to methods of dispersion compensation that may be performed in the electronic domain, using processing at the transmitting and/or receiving end of an optical link, and in particular discloses a method utilising block coding of digital information, single sideband optical transmission, and frequency domain equalisation of the resulting received signal, in order to provide complete compensation of linear dispersion in the electronic domain. This latter approach is particularly conveniently implemented using orthogonal frequency division multiplexing (OFDM) methods for the encoding and decoding of the electrical signals.
While the levels of optical non-linearity existing in most practical transmission media, and in silica glass in particular, are relatively low, the optical intensities arising in the core of waveguide structures formed in such materials, eg single-mode optical fibres, can be very high. This is particularly true in long-haul optical transmission systems, where there is an inherent trade-off between the peak optical power (ie intensity within the single-mode fibre core) and the overall system cost. Specifically, in order to maintain a high optical signal-to-noise ratio the propagating signal power must be maintained at a sufficiently high level at the input to each optical amplifier in the system. The spacing between amplifiers may be increased by launching higher optical power into the fibre spans at the output of the transmitter, and of each amplifier. However, the use of high launch powers increases the effect of optical non-linearities, resulting in higher optical signal distortion, which ultimately limits the received signal quality, and thus the maximum transmission distance achievable before the signal must be detected, recovered and regenerated. It is therefore useful to mitigate the effects of fibre non-linearity, so that the number of optical amplifiers used within a transmission link of given lengths may be reduced, and/or to enable the total unregenerated transmission length to be increased.
Compensating for non-linear transmission impairment is generally more difficult than compensating for linear processes such as chromatic dispersion. Whereas a distributed linear process may be exactly modelled as an equivalent lumped component, and compensated using lumped elements, distributed non-linear processes cannot generally be treated as equivalent lumped components, or precisely inverted at a single point within a system. Nonetheless, in appropriate circumstances such an approach provides a useful first-order approximation enabling the effects of optical non-linearities to be somewhat mitigated. However, past attempts to apply this idea have proven to be impractical, computationally difficult, and/or have provided only relatively small improvements in received signal quality.
According to one prior art approach, it has been proposed to compensate for non-linear distortion by using substantially lumped elements composed of materials having a negative non-linear coefficient, which is an analogous approach to the use of DCF for the compensation of linear chromatic dispersion. Unfortunately, it has thus far proven impractical to manufacture and deploy components utilising materials with the required non-linear properties. An alternative prior art approach is to implement an equivalent negative non-linear effect within an electronic pre-compensation system. The difficulty with this approach is that it is initially necessary to determine the required characteristics of the effective negative non-linearity. Given a sufficiently detailed knowledge of the transmission system, including the characteristics of all of the fibre spans, and the transmitted optical power levels, it is possible in principal to compute the properties of a corresponding “inverse” fibre model. Computer simulation techniques may then be used to propagate the transmitted optical signals through the inverse system model, whereby the computed output of this model is used as the input to the real system. This approach is limited by the difficulties inherent in obtaining sufficiently accurate information regarding the actual transmission system, and also by the high computational complexity of modelling the inverse system.
Accordingly, simplified approaches to pre-compensation of optical non-linearity have been proposed. According to one such approach, a constant optical phase shift is applied to each bit of a transmitted optical signal utilising a base-band return-to-zero (RZ) modulation format, wherein the phase shift is dependent only upon the two adjacent bits. In another proposal, a simplified calculation is utilised for the inverse system model, using only a single step of a conventional split-step fibre propagation model in order to represent up to two fibre spans.
Other prior art approaches have employed optical non-linearity compensation at the receiving end of a transmission system. These are generally relatively complex approaches utilising non-linear feedback systems, or the use of optical modulators operated in response to the received optical power in order to simulate a negative non-linear coefficient.
Accordingly, there remains a need for further alternative and/or improved methods and apparatus for compensating for non-linear effects in long-haul optical transmission systems. It is highly desirable that new techniques be developed which avoid the need for expensive, exotic and/or complex additional optical components to be deployed, and which enable computationally efficient compensation in the electrical domain. It is therefore an object of the present invention to provide such methods and apparatus, or at least to mitigate some of the aforementioned disadvantages of prior art approaches.